The present invention relates to the mounting and support arts. It finds particular application in conjunction with the mounting of large diameter anode x-ray tubes and will be described with particular reference thereto. It is to be appreciated, however, that the invention may also find application in conjunction with the mounting of x-ray tubes of other design and in other applications in which heat transfer from a cantilevered load through the mounting arrangement is required.
In early x-ray tubes, electrons from a cathode filament were drawn at a high voltage to a stationary target anode. The impact of the electrons caused the generation of x-rays as well as significant thermal energy. As higher power x-ray tubes were developed, the thermal energy became so large that extended use damaged the anode.
One way to distribute the thermal loading and reduce anode damage was to use a rotating anode. The electron beam was focused near a peripheral edge of an anode disk. As the anode rotated, the portion of the anode where x-rays were generated moved along an annular ring. Each spot along the annular path was heated to a very high temperature during the generation of x-rays and cooled as it rotated before it returned for the generation of x-rays. If the path of travel was too short, the target area on the anode would still contain sufficient thermal energy that the added thermal energy from the electron beam caused thermal damage to the anode surface. Accordingly, as higher power x-ray tubes were developed, the diameter and the mass of the anode continued to grow.
Typically,.the anode was mounted on a stem which was supported by a bearing assembly. In one technique for removing thermal energy, the bearings were contained in a copper housing which was integrally connected with a copper shank. A vacuum envelope, typically glass, surrounded the cathode, rotating anode, and bearing assembly. The copper shank extended from the vacuum envelope providing both a thermal path for removing heat energy from within the envelope and a metal mounting element for the anode and bearing assembly. Typically, the x-ray tube was mounted in a cantilevered arrangement by the copper shank.
In today's CT scanners with high gantry rotational speeds, x-ray tubes with massive rotating anodes are still mounted in a cantilevered fashion by a thermally conductive shank at one end. Although additional support is provided for the evacuated envelope, the anode and bearing assembly is supported solely by the shank. The evacuated envelope is surrounded by an oil bath for removing the excess thermal energy. Under the weight of the large anode, the shaft tends to bend or deform, allowing the anode structure to sag. Even a slight deformation or wobble alters the x-ray beam sufficiently as to be unacceptable for CT and other precision applications.
One solution was to construct the bearing housing from stainless steel or other stronger metals. However, stainless steel lacked the thermal conductivity of copper. In high energy x-ray tubes, the stainless steel and heat radiation across the vacuum taken together were inadequate to remove the generated heat.
Another solution was to use a strengthened high thermal conductivity alloy, particularly a dispersion strengthened copper alloy. In order to maintain the vacuum, it was necessary to form a vacuum seal between the vacuum envelope and the shank. This seal was commonly performed by brazing. In a glass envelope tube, the glass was connected with a Kovar metal, which, in turn, was brazed to the copper bearing housing. Although pure copper was readily brazed to form this vacuum-tight seal, the dispersion strengthened copper alloy did not braze reliably.
The present invention provides a new and improved mounting arrangement which overcomes the above-referenced problems and others.